The present invention relates to mechanical presses, and in particular to a monitorable, feedback controllable tool system for the dies and tool sets of the presses. More specifically, the tool and control system are operable and adaptable to dynamically adjust a single tool, the complete die set or the individual tool station of a multi-station die set to maintain the dimensional tolerances and thus the quality of the parts produced on the press.
Mechanical presses, such as straight side presses and gap frame presses for stamping and drawing, generally comprise a frame having a crown, a bed and a slide supported within the frame for reciprocal motion toward and away from the bed. The slide may be driven by a crankshaft and a connecting arm connected to the slide, to which is mounted the upper die. The lower die is mounted on a bolster connected to the bed. Mechanical presses are widely used for blanking and drawing operations, and vary substantially in size and available tonnage depending upon the intended use. The present invention is particularly well suited to, but not exclusive to, a conversion press for forming easy open beverage can ends where precise control of dimensional tolerances of certain operations, such as embossing and scoring, is critical. This precision dimensional control is required without using the excess tonnage (force) currently provided with the use of oversize kiss blocks in the tooling. The invention is also useful with other types of presses.
Many presses are operable with single or multiple tooling stations and this tooling or the part formed therefrom may vary during operation either from tool wear, temperature changes, or stock material variations. These variations or changes in parameters may cause distortions and/or dimensional variations in the parts produced or formed on the presses. Therefore, it is necessary to continuously monitor the parts produced and to alter or adjust the tooling and press to maintain production of acceptable or quality parts. This quality control function frequently necessitates removal of the dies or tools or some components thereof from the press and subsequent readjustment of the press for production of quality parts. Current industry practice is to provide the readjustments with the press in a static or non running condition, which may not incorporate the thermal and/or speed effects into the adjustment.
Presses, both the mechanical and hydraulic type, have been provided with various arrangements to attempt to accommodate variations in parameters associated with press operations. Included among these adjusting arrangements are die cushions, wherein a hydraulic fluid behind the tooling or die, generally the lower die arrangement, provides a hydraulic cushion. Other efforts at tool control included mechanically operated hydraulic systems, and hydraulic overload control systems, which accommodate or are operable to a maximum load exerted upon the die by the slide during the work stroke. The overload control systems only provide a means to stop the press in case of an overload.
Adjustment means for the press or tools have been devised to be responsive or operable as a function of the stroke frequency. In some cases the adjustments were based upon constant immersion depth of the upper tooling and its adjustment during press operation. Shut height adjustment by an electrical motor drive has been provided by sensing the shutheight on the fly, stopping the press and adjusting the slide in response to the monitored shutheight. However, the initial shutheight had to be known for comparison. Also known is adjustment of the shutheight provided by adjusting a hydraulic bolster control system, which adjusts the bolster, and consequently all tool stations simultaneously to a fixed height to thus adjust the shutheight.
A known lower tool control system utilizes mechanical springs for controlling the pressure on the work piece. However, no monitoring circuit is known for continuously testing the tooling load, comparing the tooling load to an optimum tool load, providing a feedback signal based on this comparison and adjusting individual tools to the optimal tool load. One instance of an attempt to control a forming force for a tool was provided in the case of a roll forming operation, where a controller-force detector is connected for determining the force exerted on a forming roller. The detector utilizes a contact arm for determining the position of the lower slide and through a look-up table compares force versus position relationships for control of the forming roll. This tool position is then compared to known force/position values for adjustment of the tooling in response to this change.
One prior art technique for tool or die adjustment varied the hydraulic pressure of a hydraulically supported, drop away bolster, which adjustment modified the back-up force and, consequently, the bolster component elongation and the operating shutheight of the press. The bolster used is described in U.S. Pat. No. 4,206,699. However, modifying the bolster pressure in this manner increases the force on the entire bolster and changes the shutheight, but in the case of multiple die station presses, the shutheight is changed for all stations, whether needed or not. Furthermore, this system is not automatic in that it relies on an inspection or monitoring of the produced parts and then manual adjustment of the press.
Historically press tooling has generally been set up or assembled by a trial and error type method. That is, the tooling would be installed in the press or, alternatively, a die was set up externally to the press and positioned in the press and the initial parts produced by the press and tool arrangement are tested or checked to determine if they are in the specification limits. The tooling and/or press are then manually adjusted to produce an acceptable part. The adjustments could be in the form of shutheight variation; shimming of the tools; in the case of multiple lane, progressive die arrangements, shimming of individual tooling stations in the die, or shimming the die set; and grinding of tools or a combination of such adjustments.
In a multiple lane progressive die arrangement, such as in a conversion press, the variation of a single tool station usually influences the remaining stations within a lane of the tool arrangement and, in fact, may influence the other lane or lanes by affecting the tipping moment within the die arrangement. Accordingly, the adjustment of the tooling to bring the operation at one of the die stations into specification limits may cause the other die stations to go out of specification.
In the lead frame press industry, the initial shutheight can be adjusted or zeroed and thereafter varied to attain the upper or lower limit of an acceptable or quality part. The selection of an initial setting may be determined by past operating practices and set to accommodate known variations based upon the above variable parameters including press speed (rpm or frequency) and thermal effects on operating shutheight. Further, changes in the part quality can, as noted earlier, vary with changes in the stock material dimensions from specifications. Variation in stock material thickness or hardness influences quality part production from a press or forming arrangement and affects the required forming load and press operation. Although stock variation is not a change in the press or tooling, it must be accommodated to maintain part production within specification limits.
Accommodation of the variations in tooling and/or press parameters while maintaining acceptable or quality part production has led to the practice of utilizing "kiss blocks", particularly in the can conversion industry for multi-lane progressive die arrangements. The kiss block is a massive positive stop block with a compressive resistance or stiffness greater than the stiffness of the press and is used to limit slide travel. The kiss block can be a single block or multiple blocks generally mounted within the tool area between the slide and bolster with a significant cross-sectional area. The kiss blocks thus define the minimum separation at bottom dead center between the upper and lower dies. Therefore, even if the press is sped up or there is a change in the thermal equilibrium, which generally causes elongation or thermal expansion of the mechanical connections and thus less separation between the tools than in their unrestrained state, the kiss blocks limit further shutheight change of the press. However, the use of oversized kiss blocks to limit the travel of the slide can produce very severe stresses and loads on the press. Typically in the conversion industry, when it is determined that the score line depth is insufficient, the load on the press is increased by decreasing the operating shutheight, but limiting the travel of the upper tooling through the use of kiss blocks. The press experiences a mechanical over-travel condition, however, and the tools will continue to travel only to the limit of the kiss block deflection, which maintains the part dimensional specifications. This practice puts a severe strain upon the press frame, and results in an excess work function by the press, which work or energy is not applied to nor required for formation of the stock material to its finished shape. Therefore, this practice results in lower press life; more frequent press breakdowns, which implies less press reliability; and, in addition, requires recess energy not applied to product formation.
Indicative of the above problems is that condition which is found in the can-end industry, particularly for the production of ecologically acceptable can ends or can ends with tear tabs. The press arrangements are generally multi lane, multi station arrangements that are subject to very close tolerances and high volume production rates. Thus it can be appreciated that these high volume rates require high-speed press operation, which results in relatively high or elevated temperature on the tool and pres elements. The stock material is relatively thin but will vary in thickness and/or hardness during the production run. High rate press operation results in tool wear, which may result from erosion, jamming a t a particular tool station or any other condition which changes the profile and dimension of the tool. These end conversion tools are typically reset on an individual tool station basis, whereas progressive dies are typically removed from the press and rebound as a unit. The problem with replacing only one worn tool is that the remaining tools will also have experienced wear. Replacement of a worn or broken tool with a new tool, therefore, can disrupt or disturb the load balance in the die set, causing a potential loss of overall part quality and production.